1. Field of the Invention
The present invention relates to alkaline storage batteries. Particularly, the present invention improves an inexpensive sintered iron substrate having three-dimensionally intercommunicating spaces and provides inexpensive alkaline storage batteries using this substrate.
2. Description of Related Art
Due to the recent rapid progress of portable and cordless devices, a demand for small-sized and light-weight secondary batteries having high energy density is increasing as electric sources of these devices. In the market, inexpensive secondary batteries of high capacity are especially in demand.
Therefore, a reduction in cost of alkaline storage batteries represented by nickel-cadmium storage batteries and nickel-metal hydride storage batteries is strongly demanded.
As electrodes for alkaline storage batteries, there are used spongy substrates which are mainly composed of pure nickel having three-dimensionally intercommunicating spaces and which are filled with an active material. However, the substrates are so expensive that the cost of the substrates occupies about 1/4 of the cost of raw materials and components for the batteries.
In order to reduce the cost of raw materials of alkaline storage batteries, U.S. Pat. No. 5,681,673 proposes an electrode comprising an iron powder sintered substrate plated with nickel on the surface and having three-dimensionally intercommunicating spaces, said substrate being filled with an active material, as a substitute for the substrate comprising pure nickel mentioned above.
This iron powder sintered substrate is produced by coating an iron powder slurry on a synthetic resin core body, for example, a foamed polyurethane resin core body, and then heat treating the coated core body. By this heat treatment, the foamed polyurethane resin core body is removed and simultaneously the remaining iron powders are sintered. Subsequently, the surface is plated with nickel.
However, in the case of the above-mentioned iron powder sintered substrate, the iron base is not completely covered by the nickel plating because of its complicated three-dimensional structure. Therefore, there are certain portions which are not plated with nickel (pin hole portions).
For this reason, in an alkaline storage battery constructed using this iron powder sintered substrate, iron dissolves into an alkaline electrolyte from the portions of the substrate not plated with nickel (pin hole portions), namely, iron-exposed portions, which results in adverse effects on battery characteristics after initial charge and discharge and after long-term storage.
In order to form an oxide of iron on the iron-exposed portions as a protective film, it is necessary to diminish oxygen overvoltage at the time of charging to reduce charging efficiency. This procedure also causes a decrease of the utilization ratio of the active material, and, hence, no sufficient output can be attained for a battery.